In this study, the Salmonella/microsome assay, using the micro-suspension method, was utilized to evaluate water for public supply at three sites in Rio Grande do Sul. The first site selected was in an area under industrial influence and the others were in non-industrial reference areas. Based on 40 L samples of raw water and/or after conventional treatment, compounds were extracted with XAD4 resins using natural and acidic pH, and the extracts were analyzed in the TA98 and TA100 strains with and without S9. Raw water extracts in the industrial region induced 27.4 revertants/L (rev/L) for TA100+S9 up to 226.3 rev/L for TA100-S9, both for acidic pH extracts. After conventional treatment the responses varied from 20.6 rev/L (TA98-S9) for natural pH extracts to 755.5 rev/L (TA98-S9) for acidic pH extracts. For acidic extracts obtained from reference site samples, the response, with metabolic activation only, ranged from negative to minimal. Direct mutagenic responses in acidic extracts may be elevated in treated extracts, compared with raw water extracts, influenced by the presence of by-products of the chlorination process. However, the mutagenicity observed in the treated water extracts in the industrial area increased and reflected mainly the combination of directly and indirectly acting compounds in the source waters, that are heavily influenced by anthropogenic factors.